This invention relates to an electro-optical device, and in particular to a liquid crystal display device which includes an optically anisotropic layer.
In a conventional homogeneous ECB liquid crystal device a twisted nematic liquid crystal device, or a super twisted nematic liquid crystal device, display is effected by control of the double refraction of liquid crystals. Thus, coloration of the display is inevitable. Japanese Patent Application No. 62-121701 attempts to avoid coloration by providing an optical anisotropic substance in addition to a liquid crystal display cell. This is referred to as an "NTN mode" liquid crystal display device.
A typical art NTN mode electro-optical liquid crystal device is shown in cross-section in FIG. 3. Electro-optical device 150 includes an upper polarizer 101, a liquid crystal cell 102, an optically anisotropic layer 104 and a lower polarizer 105. Liquid crystal cell 102 includes a liquid crystal material 124 disposed between an upper substrate 121 having transparent electrodes 123 disposed thereon and a lower substrate 122 with transparent electrodes 125 disposed thereon. A spacer 126 serves to hold substrates 121 and 122 apart.
ZLI-4287 (.DELTA.n=0.0929) (product of Merck Co.) is used as liquid crystal material 124 in liquid crystal cell 102 and it is homogeneously oriented in cell 102 having a cell gap (d) of 5.9 .mu.m. The retardation (.DELTA.n.times.d) is 0.55 .mu.m.
Optically anisotropic layer 104 is a monoaxially stretched film of, for example, a polycarbonate having refractive indices of N1o=1.589, N2o=1.590 and N3e=1.594. The film thickness is 140 .mu.m, and the retardation is 0.56 .mu.m. N3e is the refractive index in the stretched direction of the film or the direction of the optical axis. A conventional monoaxially stretched film has a refractive index of an extraordinary ray (Ne) which is larger than the refractive index of an ordinary ray (No). Thus, the optically anisotropic substance is said to have an optically positive uniaxiality.
FIG. 4 illustrates the relationship of the respective axes of a conventional electro-optical liquid crystal device including a monoaxially stretched film. The polarization axis (absorption axis) 100 of upper polarizing plate and the rubbing direction 210 of upper substrate 121 of liquid crystal cell 102 form an angle 912 of 45.degree.. The angle 942 between the stretch direction 400 of the monoaxially stretched film and the rubbing direction 220 of lower substrate 122 of the liquid crystal cell is 90.degree.. An angle 954 between the polarization axis (absorption axis) 500 and stretched direction 400 of lower polarizing plate 105 is 45.degree..
FIG. 5 illustrates the electro-optical characteristics of electro-optical liquid crystal device 150 of FIG. 3. Curves 61, 62 and 63 are voltage-transmittance curves for light rays having wavelengths of 450 nm, 550 nm, and 650 nm, respectively, applied in a direction vertical to the plane of the panel. It is evident from FIG. 5 that the transmittance values are low in the OFF state in each case. Thus, electro-optical liquid crystal device 150 has excellent characteristics for displaying high contrast images when observed from a direction vertical to the plane of the panel.
However, conventional NTN mode-type electro-optical liquid crystal devices have a narrow visual angle range ("visual angle") in which the displayed image may be viewed satisfactorily. FIG. 7 illustrates the visual angle characteristics of electro-optical liquid crystal devices in such as device 150. The center 80 of FIG. 7 represents the direction vertical to the panel plane, and outer circles 81, 82, 83, 84 and 85 represent inclined angles of 10.degree., 20.degree., 30.degree., 40.degree. and 50.degree., respectively, in the direction vertical from the center of the circles. The four up, down, left and right directions in FIG. 7 correspond to the four directions shown in FIG. 4. Curves 71, 72, 73, 74 and 75 represent isocontrast curves having contrast ratios of 1, 3, 10, 30 and 100, respectively. The shadowed region has a contrast ratio of 1 or less, in which the display is reversed.
As mentioned above, conventional NTN mode-type electro-optical liquid crystal devices suffer from a narrow high contrast region and an easily reversed display. The reversed display psychologically contributes significantly in causing the sense of a narrow visual angle.
Accordingly, it is desirable to provide an improved electro-optical liquid crystal device which eliminates these problems associated with conventional devices and provides a high contrast image with a broad viewing angle.